1. Field of the Invention
The present invention relates to a signal processor, and more particularly to a signal processor for use in an internal combustion engine.
2. Description of the Background Art
Among signal processors for use in internal combustion engines or the like, an analog input signal processor having the function of adjusting both the gain and frequency characteristics of the input signal processor has publicly been known, which is shown in, for example, Japanese Patent Application Laid-Open No. 2002-16460 (FIG. 1 and Abstract). JP 2002-16460 describes an invention related to a gain control circuit, and presents the concept of changing the switching duty ratio of switching devices connected in parallel or in series to a resistor that determines the gain of an operational amplifier, thereby adjusting the gain as well as controlling frequency characteristics of a filter in an alternating current amplifier.
A switched capacitor filter circuit is widely in practical use as a component of a filter circuit, which is shown in, for example, Japanese Patent Application Laid-Open No. 11-205113 (1999) (FIG. 11 and paragraphs 0002 to 0013). JP 11-205113 describes an invention related to a switching circuit and a switched capacitor filter circuit, and presents the concept of charging/discharging a capacitor having a capacitance C1 in a variable cycle Ts, thereby obtaining an equivalent variable resistance where a resistance value R is expressed as Ts/Cl.
Further, Japanese Patent Application Laid-Open No. 2002-130043 (FIG. 1, paragraphs 0017 and 0018) describes an invention related to a signal processor for use in an internal combustion engine or the like, and presents the concept of a knock detector for an engine provided with a switched capacitor filter circuit constituting a band-pass filter, a variable gain amplifier circuit and a peak hold circuit.
Furthermore, Japanese Patent Application Laid-Open No. 5-306645 (1993) (FIG. 11 and paragraph 0044) describes an invention related to a knock detector for an internal combustion engine, and presents the concept of adjusting the signal-passing frequency bandwidth of a switched capacitor filter circuit constituting a band-pass filter in accordance with operating conditions of the internal combustion engine.
The gain control circuit described in the above-mentioned JP 2002-16460 does not involve the concept of changing switching frequencies of the switching devices. That is, JP 2002-16460 describes that changing the switching duty ratio of the switching devices causes the gain and frequency characteristics of the filter to be varied in synchronization with each other, so that the maximum gain and frequency characteristics cannot be varied independently.
In the signal processor described in the above-mentioned JP 2002-130043 or 5-306645, signals for varying the filter characteristics and for varying the gain characteristics, respectively, are separated and supplied independently. That is, either JP 2002-130043 or 5-306645 requires a control part to supply two types of control signals.
Further, in the case where it is desired to increase, for example, the maximum gain or filter characteristics of an input signal processor by 10% in a signal processor for use in an internal combustion engine or the like, the maximum gain or filter characteristics exhibit an increase ranging from 20 to 0% if there is an error of 10% due to fluctuations in numeric value in circuit components. This causes a problem in that the purpose of improvements is not achieved. In the case where a more delicate adjustment is required, the influence of fluctuations in numeric value in circuit components will be a more serious drawback. Therefore, in adjusting the gain or filter characteristics of an input signal processor, correcting fluctuations in numeric value in circuit components is a realistic challenge, and the gain and frequency characteristics of an input signal processor are closely related to each other.